1. Field of the Invention
This invention relates to a process for bending a metal sheet, and more particularly to a process for bending a metal sheet through a desired angle, with the inner bend radius being less than the thickness of the metal sheet, without causing damages such as cracking in a bend.
2. Description of the Prior Art
Known as one of processes for bending a metal sheet is one which uses a press having a punch and a die.
According to the above process, a metal sheet may be bent through a desired inner bend radius i.e., an inner radius of curvature for a desired bend. However, the above bending process suffers from a shortcoming that upon bending, the neutral plane, i.e., an imaginary neutral plane of a metal sheet, where neither stretching nor compression takes place, tends to shift inwardly of a bend, as an inner bend radius decreases. For this reason, the bending of a metal sheet according to a press through inner bend radius less than the thickness of a metal sheet results in an increase in elongation of a material of the metal sheet on its outer side of a bend, causing damages such as cracking and the like in the outer surface of a bend. This is particularly true with the bending of a metal sheet which affords low ductility, such as for instance, a high tensile rolled steel plate. Thus, considerable difficulty is encountered with the bending of a metal sheet of a low ductility through an inner bend radius less than the thickness of a metal sheet, while preventing damages in a bend of the sheet.